This is a proposal to study local cerebral blood flow, blood volume and oxygen metabolism in a total of 50 subjects: 10 patients with mild SDAT, 15 with questionable SDAT and 25 age-matched controls. The subjects will be seen for the first time in years 01 or 02 of the project, and then once again 24 months later. Three measurements are to be made at each examination: (1) cerebral blood flow (CBF) with 15 O-labeled H20, (2) cerebral blood volume (CBV) with 15 O-labeled carbon monoxide, and (3) cerebral oxygen extraction with 15 O-labeled 02. Cerebral metabolic rate for oxygen (CMRO 2) will be calculated as the product of CBF, 02 extraction and arterial oxygen content. The 15 02 is administered as a single breath; C 15 O-labeled red cells and H2 15 O are by IV injection. The primary goal of these studies is to compare normal aging and SDAT with respect to cerebral hemodynamic and metabolic variables, both cross-sectionally and longitudinally over a 2-year interval. Particular attention will be paid to the balance between cerebral metabolism and blood flow (revealed by 02 extraction) as an indicator of the presence or absence of cerebral ischemia. Although blood volume measurements are utilized in the computation of CMRO 2, they will also be used to confirm a suggestion from the investigator's earlier work that CBV is increased in demented patients.